1. Field of the Invention
This invention relates to the p-type doping of optical materials formed from Group II-VI combinations in general, and more particularly to the p-type doping of HgCdTe using molecular beam epitaxy.
2. Description of the Related Art
The ability to impart a p-type doping to HgCdTe is critical to the production of junction structures which are at the heart of infrared (IR) detectors. However, a suitable doping technique using molecular beam epitaxy that is compatible with preferred fabrication processes for IR detectors has not previously been found.
HgCdTe is difficult to prepare for use in detection devices by either bulk or epitaxial techniques. The most commonly used epitaxial growth process for this material has been liquid phase epitaxy. Although high performance infrared detectors have been realized with growth by liquid phase epitaxy, the technique cannot produce abrupt hetero-junctions and superlattices required for advanced opto-electronic devices. A review of various growth techniques is provided in J. P. Faurie, et al., "Latest Developments in the Growth of Hg.sub.1-x Cd.sub.x Te and CdTe-HgTe Superlattices by Molecular Beam Epitaxy", J. Vac. Sci. Technol. A, Vol. 1, No. 3, July/September 1983, pages 1593-97.
The molecular beam epitaxy (MBE) technique, on the other hand, is suitable for the growth of high quality epilayers, abrupt hetero-junctions and alternate microstructures such as superlattices. This technique is described in J. P. Faurie, et al., "Molecular Beam Epitaxy of II-VI Compounds: Hg.sub.1-x Cd.sub.x Te", J. Cryst. Growth, Vol. 54, No. 3, pages 582-585, 1981.
MBE is a vacuum deposition process. Several effusion cells are used, each cell comprising an electrically heated crucible containing one of the substances of the compound to be grown. Upon heating, the cells produce atomic or molecular beam fluxes of mercury, cadmium, tellurium or CdTe. The fluxes are directed onto the surface of the substrate, where they react with each other and produce an epitaxial layer.
Arsenic (As) is commonly used as a p-type dopant, although antimony (Sb) and Phosphorus (P) are also available for this purpose. However, it has been previously discovered that these Group V elements can also act as n-type dopants in HgCdTe, rather than p-type dopants, when they are incorporated using the MBE process. See M. Boukerche, et al., "The Doping of Mercury Cadmium Telluride Grown By Molecular Beam Epitaxy", J. Vac. Sci. Technol. A, Vol. 6, No. 4, July/August, 1988, pages 2830-33. The n-type doping is believed to result from the occupation of Hg vacancies in the metal lattice by As. It is difficult to adjust the vapor pressures of Cd and Hg to assure that all positions in the metal lattice are filled. Since Te has a higher sticking coefficient than Hg, there will be a tendency towards an excess of Hg lattice vacancies. The Hg vacancies facilitate the substitution of As atoms in the metal lattice for Hg, preventing them from exclusively entering into the Te sublattice in the alloy. This effect is enhanced by the fact that As and Te form a compound similar to the one formed between Cd and Te.
As has a deficiency of one electron compared to Te, and therefore becomes a p-type dopant when substituted into the Te position in the lattice. However, As has an excess of three electrons compared to Group II materials such as Hg and Cd, and therefore acts as an n-type dopant when bound into the Hg vacancies in the Hg-Cd (metal) lattice in the HgCdTe layer. This phenomenon has effectively precluded the practical use of As and other Group V elements as p-type dopants for HgCdTe in connection with normal MBE.
One approach towards a solution to this problem might be to increase the Hg and/or Cd vapor pressure to eliminate, or at least reduce, the metal lattice vacancies. However, increasing the vapor pressure much above 10.sup.-6 atmosphere imbalances the stoichiometry, and in general would exceed the low vapor pressure regime required for effective MBE. Higher vapor pressures cause the molecular beams to scatter, and reduces the control achievable over the materials reaching the substrate.
Another approach has been to avoid the use of As altogether, and instead employ silver (Ag, Group I). See, M. L. Roge, et al., "Controlled P-Type Impurity Doping of Hg.sub.1-x Cd.sub.x Te During Growth By Molecular-Beam Epitaxy", J. Vac. Sci. Technol. A, Vol. 6, No. 4, July/August, 1988, pages 2826-29. While avoiding the n-type doping problem, this technique also eliminates the distinct advantages of As used as a dopant, especially because Ag is a fast diffuser in II-VI compounds.
The use of photoassisted doping with As has been another avenue of exploration to overcome the n-type doping problem. This involves modifying the normal MBE system to permit illumination of the substrate during the growth process and is described, for example, in S. Hwang, et al., "Properties of Doped CdTe Films Grown By Photoassisted Molecular-Beam Epitaxy", J. Vac. Sci. Technol. A, Vol. 6, No. 4, July/August, 1988, pages 2821-25. This technique adds an extra degree of complexity to the system, and has not yet been demonstrated to be fully effective in the alloy system.
The situation has been described thus far in terms of HgCdTe. It should be understood that similar considerations apply to HgTe and CdTe. Similar doping by Group V elements will also be encountered with other Group II-VI combinations. In general, Zn, Cd, Hg and Mg from Group II can be combined with S, Se, and Te from Group VI and are suitable for MBE fabrication techniques, the exact combination selected depending upon the desired bandgap and the kinetics of growth and dopant incorporation in the lattice.